1. Field of the Invention
This invention relates to the field of low, or zero, insertion force connectors, particularly to receive the edges of printed circuit cards and the like.
2. The Prior Art
Printed circuit cards, or boards, are normally relatively thin sheets of insulating material with conductive patterns formed on one surface, or usually both surfaces, by a printing technique that normally involves a photographic printing process. The conductive patterns may be electrical circuit components, such as inductors, capacitors, or resistors, but usually the patterns only form connecting links between specific circuit locations to which separately formed electrical components can be connected, usually by a soldering process.
Printed circuits, however formed, usually are arranged so that the terminals of the circuit are formed along a straight edge of the board to be inserted into a multi-conductor connector. This connector is mounted on another board, which is commonly called a mother board, and the board inserted into that connector is commonly called a daughter board. Some printed circuit boards are large enough to have over two hundred connections along the edge. If each of the connections must be inserted into good conductive relationship with a separate conductor in the multi-unit connector, and if each of the conductors exerts a half-pound pressure on the printed circuit board to be certain of making good electrical contact, the total pressure may therefore be more than 100 pounds. Although the pressure is perpendicular to the surface of the printed circuit board, it exerts a frictional force that interferes with movement of the board either into the connector or out of the connector, and the magnitude of such frictional force may be of the same order of magnitude as the total force exerted perpendicular to the surface of the board, depending on the coefficient of friction between the connectors and the surface against which they press.
There has long been a desire on the part of those in the electronics industry to have a connector capable of accepting the edge of a printed circuit board easily but with means for increasing the force exerted by the connector on the edge of the board after insertion. Most desirably, the connector should exert zero force while the edge of the printed circuit board is being inserted into the connector but should exert at least the normal force after insertion. This would permit a printed circuit board having as many contacts along its edge as is otherwise feasible to be inserted into a connector of corresponding length and having the appropriate number of connectors. Furthermore, it would make it possible to slide the printed circuit board into the connector along the longitudinal direction of the connector, rather than perpendicular to the longitudinal direction. This would make it possible to arrange connectors along opposite edges of a printed circuit board or even along three or all four edges of a printed circuit board.
Zero insertion force connectors have been proposed hereto fore in which an elongated cam member extending longitudinally along the connector is pivotal about an offset axis to exert a cam force that is in the proper direction to move all of the conductors of the connector simultaneously toward or away from the gap in which the printed circuit board is inserted. One of the disadvantages of such pivotally arranged devices is that they must be insulated from the individual conductors so as not to short circuit the conductors together. Another disadvantage is that, if such elongated cams are too long, the cumulative force of the conductors will make it necessary for the elongated cam to twist along its length, thereby causing each of the conductors to make contact and to break contact with the respective part of the printed circuit board at a time slightly different than the time of the next adjacent conductor. The torque that must be exerted to rotate the elongated cam is the sum of all conductor cam friction moments because of simultaneous actuations.